BS34 Mixture Screws on KZ1100 Ltd

Hopefully a very simple question re : mixture screws on my Mikuni BS34 carbs. Am I right to think that screwing in leans out the mixture. I've recently stripped, cleaned and rebuilt the carbs, setting them to 2 turns out from bottoming to start with. Bike is starting and idling well, and also responds well to the throttle. When I stripped the carbs they were 3-1/2 turns out and the bike started and idled well but would bog down when the throttle was applied slowly.

So... can anybody enlighten me on this mixture screw question? I'll tell you why I've "confused" myself, its because over many years I've got used to the mixture screw on a carb being turned in clockwise to reduce fuel and therefor lean out the mix. The amount of info and disinformation on the web on this doesn't help either, its that word again...confusing...with varying opinions.

From what I can gather this is called an "air" screw so surely that would work the opposite way round, anti clockwise to increase air flow. What's also confusing me and making me doubt myself is the fact that this mixture screw is on the engine side of the carb so seems to lend itself to adjusting fuel volume. Turn clockwise to reduce fuel. Why plug it originally if its just to increase air. By the way, there isn't an air jet in the throat end of these BS34 carbs unlike others I've seen.

That's an awful lot of words above to ask about a mixture screw, just call me a dumb cluck and put me out of my misery please guys...

IN = Lean
OUT = Rich

May visualize from this BS34 diagram how turning the pilot screw in clockwise reduces the volume of mixture through the orifice into the carb bore (which produces a leaner mixture into the combustion chamber), and how turning the pilot screw out counter-clockwise increases the volume of mixture through the orifice into the carb bore (which produces a richer mixture into the combustion chamber).




I concur with your preferred terminology, that the adjustment screws at hand are indeed pilot mixture screws, and not pilot air screws which are typically located on the air box side of carbs.

A pilot air screw governs the amount of air being allowed to reach the pilot jet, whereby screwing in clockwise reduces air and thereby enrichens the pilot circuit; and screwing out counter-clockwise increases air and thereby leans the pilot circuit.

The following illustration compares the different pilot circuits in manual slide carbs, being side-located air screw on air box side of carb, and bottom located mixture screw on engine side of carb.



Good Fortune!

Thanks for the answers guys, I really do appreciate it. It really was a simple question, but doubt had crept in reading Mikuni articles on the web. A bit like getting a couple of miles down the road and wondering if you'd left the kettle on the gas ring. You know you haven't but start to convince yourself that you have....

Thanks for those diagrams Patton. Looking at those I can see the operation of the system clearly. A picture is worth a thousand words eh!

Bike is running smoothly and pulls like a train with the screws 2-1/2 turns out.